The present invention relates to a disc drive microactuator, and more particularly to a high resolution positioning mechanism implemented at the slider level for selectively moving a transducer portion of the slider radially (laterally) and vertically with respect to circumferential data tracks of a rotatable disc.
As the areal density of concentric data tracks on magnetic discs continues to increase (that is, the size of data tracks and radial spacing between data tracks are decreasing), more precise head positioning is required. Head positioning in a hard disc drive includes two distinct but related aspects: tracking control (i.e., radial positioning of the head) and fly-height control (i.e., head-media spacing). As discussed herein, both aspects are important considerations for the hard disc drives in the future.
Conventionally, tracking control (radial head positioning) is accomplished by operating an actuator arm with a large-scale actuation motor, such as a voice coil motor, to radially position a head on a flexure at the end of the actuator arm. The large-scale motor lacks sufficient resolution to effectively accommodate high track-density discs. Thus, a high resolution head positioning mechanism, or microactuator, is necessary to accommodate the more densely spaced tracks.
One promising approach for high resolution head positioning involves employing a high resolution microactuator in addition to the conventional lower resolution actuator motor, thereby effecting head positioning through dual-stage actuation. Various microactuator designs have been considered to accomplish high resolution head positioning. Some designs are employed to deform disc drive components such as the actuator arm or the flexure in order to achieve minute displacements by bending. Other designs introduce a separate microactuator component at an interface between disc drive components. U.S. Pat. No. 6,118,637 to Wright et al., for example, discloses an assembly including a gimbal, a piezoelectric element bonded to the gimbal and electrically connected to a voltage source, and a slider connected to the piezoelectric element. In the Wright patent, the microactuator (the piezoelectric element) is a separate unit that operates to change to position of the entire slider.
While many previous microactuator designs are able to deliver satisfactory micropositioning performance, their effectiveness is inherently limited by the sheer mass that the microactuators are designed to move. In order to move or bend one or more of the disc drive components, the microactuator employed must provide a relatively large amount of force, which requires either a complex or relatively massive microactuator motor mechanism.
More recent developments include building a microactuator at the slider level, that is, to have a microactuating means built in the slider to positionally adjust a part of the slider instead of the entire slider. For example, a microactuator for moving a transducer-carrying portion of the slider is disclosed in U.S. application Ser. No. 09/733,351, filed Apr. 5, 2000, entitled xe2x80x9cSlider-Level Microactuator for Precise Head Positioningxe2x80x9d. The slider according to that patent application includes a main body carried by a flexure. A stator extends from the main body, and a plurality of beams extend from the stator, the beams being flexible in a lateral direction. A rotor is connected to the stator by the plurality of beams, forming a gap between the stator and the rotor. The rotor carries the transducing head. A plurality of stator electrodes are formed on the stator, and a plurality of rotor electrodes are formed on the rotor to confront the stator electrodes across the gap. The resultant electrostatic microactuator is able to laterally move the rotor within the slider. The above identified U.S. application is hereby incorporated herein by reference.
Compared to the tracking control, fly-height control is a separate but related problem. In addition to having direct impact on radial positioning resolution, a higher areal density of data tracks also requires that the fly-height be decreased in order to afford higher signal resolution. That is, it is desirable for the air-bearing surface of a slider to fly as close to the media as possible, without touching the media to produce better resolution of data on the media, because read/write signal strength is dependent on the distance between the magnetic imaging gap in the read/write head, and close spacing improves transducer performance without having to improve sensitivity of the transducer.
As the fly-height of the head decreases, fluctuation, vibration, roughness of the disc surface and thermal effects start to play an increasingly important role, creating a more stringent requirement for fly-height stability which can be effectively addressed only using an active fly height control mechanism.
In addition to the random fluctuation caused by mechanical noises, it is known that the air flow, which causes the slider to float, increases as the head is moved from the inner to the outer circumference of the disc. An adjustment device is thus required in order to keep the height at which the head slider is floating constant as the radial positioning the head slider above the disc surface changes.
Furthermore, because a typical head in a hard disc drive is actually utilized only in a small percentage of the time when the hard disc drive is operating, there is a need for active fly-height control to reduce wearing on the head and the disc by avoiding or reducing the head-disc contact during turning of the disc. As a related issue, most present slider assemblies use a positive loading mounting system which is configured to rest the slider upon the magnetic media disc when the disc is not turning, allowing the slider to xe2x80x9cfly abovexe2x80x9d the disc after it begins to turn. With positive loading, a slider is biased toward the disc; its air-bearing surface rides above the disc only after the viscous air currents are developed by rotation of the disc. A potential problem of positive loading is that the heads and slider may stick to the disc when it has stopped due to formation of a xe2x80x9cvacuumxe2x80x9d weld between the opposed precision flat surfaces of the slider and disc. It is therefore desirable to have active control over the vertical position of the slider to avoid or help to break a vacuum weld.
Various methods exist in the prior art for controlling transducer head fly-height. For example, it is known to address the head-media spacing loss due to thermal expansion of the transducer by optimizing the thermal mechanical structure and properties of the transducer. Such a method is in essence a passive countermeasure and fails to actively adjust the pole tip position of the transducer to consistently minimize its impact on head-media spacing.
Several patents discuss the use of piezoelectric material in a slider, to adjust the position of a transducer mounted to the slider. For example, U.S. Pat. No. 5,021,906 (Chang et al.) discloses a programmable air bearing slider with a deformable central region between leading edge and trailing edge regions. The deformable region is controlled electrically to change the angle between the leading and trailing regions, thus to change the position of a transducer mounted to the trailing region. The deformable central region is made of an electrically deformable material (such as a piezoelectric material) while the leading region and the trailing region are made of conventional ceramic materials for making sliders. The patent makes no disclosure as to how the slider and the deformable central region are made and assembled.
U.S. Pat. No. 5,991,113 (Meyers et al.) discloses a transducer flexible toward and away from the air bearing surface responsive to changes in the slider operating temperature. Transducer movement is either due to a difference in thermal expansion coefficients between a transducing region of the slider incorporating the transducer and the remainder of the slider body, or by virtue of a strip of thermally expansive material incorporated into the slider near the transducer to contribute to the displacement by its own expansion. To achieve such transducer movement, special materials of different thermal expansion characteristics are used to make the area carrying the transducer and the rest of the slider. In addition, a heating device and a temperature control circuit are required.
U.S. Pat. No. 4,605,977 (Matthews) discloses a flexible beam affixed to the slider providing a cantilever structure. A magnetic head is mounted on the free end of the cantilever. A pair of oppositely polarized piezoelectric crystals are mounted on the cantilever assembly. When energized by an electrical driving source, the cantilever beam is flexed upwards or downwards thereby changing the distance of the magnetic head from the disc. The beam in the above patent, however, is separately machined from a material and then attached to the slider.
The problems which exist in the prior art schemes for fly-height control include difficulties in fabrication, large activation voltages required for deforming materials a sufficient amount to control the transducer position, lack of fast response bandwidth required in disc operation, lack of simplicity of implementation, and most of all, lack of an integrated slider level microactuating scheme to provide active head positioning having both tracking control and fly-height control.
The present invention provides a microfabricated slider assembly in a disc drive for supporting a transducer head with respect to a rotatable disc. The slider assembly comprises a slider body having a stator and a rotor, wherein the rotor and the stator are integrally formed from a single substrate, the rotor has a head supporting portion carrying the transducer head, and the head supporting portion of the rotor is movable in both vertical and lateral directions in relation to the stator. The movement of the head supporting portion of the rotor in the lateral direction is actively controlled by a microactuator. The movement of the head supporting portion of the rotor in the vertical direction may be passive only, but can optionally be actively controlled by the same or a second microactuator.
In one embodiment of the inventive slider assembly, the rotor has a head supporting portion and a base portion. The base portion of the rotor is movable along with the head supporting portion of the rotor in relation to the stator, and the head supporting portion of the rotor is further movable in relation to the base portion of the rotor. As a combined effect, the head supporting portion of the rotor is capable of moving in both lateral and vertical directions in relation to the stator.